Tachyarrhythmias are abnormal heart rhythms characterized by a rapid heart rate. Examples of tachyarrhythmias include supraventricular tachycardias such as atrial tachycardia and atrial fibrillation. The most dangerous tachyarrhythmias, however, are ventricular tachycardia and ventricular fibrillation. Ventricular rhythms occur when an excitatory focus in the ventricle usurps control of the heart rate from the sinoatrial node. The result is rapid and irregular contraction of the ventricles out of electromechanical synchrony with the atria. Most ventricular rhythms exhibit an abnormal QRS complex in an electrocardiogram because they do not use the normal ventricular conduction system, the depolarization spreading instead from the excitatory focus directly into the myocardium. Ventricular tachycardia is characterized by distorted QRS complexes occurring at a rapid rate, while ventricular fibrillation is diagnosed when the ventricle depolarizes in a chaotic fashion with no recognizable QRS complexes. Both ventricular tachycardia and ventricular fibrillation are hemodynamically compromising, and both can be life-threatening. Ventricular fibrillation, however, causes circulatory arrest within seconds and is the most common cause of sudden cardiac death.
Cardioversion (an electrical shock delivered to the heart synchronously with the QRS complex) and defibrillation (an electrical shock delivered without synchronization to the QRS complex to terminate ventricular fibrillation) can be used to terminate most tachycardias. The electric shock terminates the tachycardia by depolarizing all excitable myocardium to render it refractory to further excitation. Implantable cardioverter/defibrillators (ICD's) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation is detected by the device.
Another type of electrical therapy for tachycardia is antitachycardia pacing (ATP). In ATP, the heart is competitively paced with one or more pacing pulses in an effort to interrupt reentrant circuits causing the tachycardia. Modem ICD's have ATP capability so that ATP therapy is delivered to the heart when a tachycardia is detected, while a shock pulse is delivered when fibrillation occurs. Although cardioversion/defibrillation will terminate tachycardia, it consumes a large amount of stored power from the battery and results in some patient discomfort owing to the high voltage of the shock pulses. It is desirable, therefore, for the ICD to use ATP to terminate a tachyarrhythmia whenever possible. Generally, only cardioversion/defibrillation will terminate fibrillation and certain high rate tachycardias, while ATP can be used to treat lower rate tachycardias.
Cardioversion/defibrillation and anti-tachycardia pacing may also be used to treat atrial tachyarrhythmias, such as atrial fibrillation and atrial flutter. These tachyarrhythmias arise from excitatory foci in the atria. Although not immediately life-threatening, it is important to treat atrial fibrillation for several reasons. First, atrial fibrillation is associated with a loss of atrio-ventricular synchrony which can be hemodynamically compromising and cause such symptoms as dyspnea, fatigue, vertigo, and angina. Atrial fibrillation can also predispose to strokes resulting from emboli forming in the left atrium.
In current ICD's with ATP capability, ventricular fibrillation is distinguished from ventricular tachycardia using rate based criteria so that ATP or shock therapy can be delivered as appropriate. The heart rate is usually measured by detection of the time between successive R waves (i.e., ventricular depolarizations). A measured heart rate is classified as a tachycardia when the rate is in a ventricular tachycardia zone, defined as a range of rates above a specified tachycardia detection rate but below a specified fibrillation detection rate. A measured heart rate above the fibrillation detection rate is in the ventricular fibrillation zone and is classified as a fibrillation.
It is common in cardiac rhythm management devices with both atrial and ventricular sensing channels for the atrial sensing channel to be blanked after a ventricular sense for a specified blanking interval. This is done to avoid far-field sensing of ventricular depolarizations by the atrial sensing channel. If the ventricular rate is high, however, this may lead to undersensing of atrial depolarizations. Consequently, the atrial rate will be underestimated. This may lead to the device delivering ventricular anti-tachycardia therapy when a more appropriate therapy would be atrial anti-tachycardia therapy.